1. Field of the Invention
The present invention pertains to an image forming apparatus using the electrophotographic method, and more particularly, to an image forming apparatus in which a transfer control method has a unique feature.
2. Description of the Related Art
In an image forming apparatus using the electrophotographic method, for the transfer control used when a toner image that is obtained by developing a latent image formed on the photoreceptor by means of toner is transferred to a transfer medium, a fixed-voltage transfer control method or a fixed-current transfer control method is used. The fixed-voltage transfer control method is a method that performs control such that the detected transfer voltage level is maintained at a prescribed voltage level. The fixed-current transfer control method is a method that performs control such that the detected transfer current level is maintained at a prescribed current level.
In Japanese Laid-Open Patent Application Hei 4-258980, an image forming apparatus is disclosed which performs transfer control by detecting the output current level to perform calculation and by obtaining the target output voltage level based on the result of calculation and the output voltage level.
The image forming apparatus disclosed in Japanese Laid-Open Patent Application Hei 5-289463 determines the transfer current immediately before the print sequence from the charge level of the toner developing area, and detects the resistance of the transfer medium before it reaches the transfer area. The transfer charge level is then estimated from the product of this resistance and the transfer current. In other words, it discloses a system in which image forming parameters such as discharge, charge, exposure, development, etc., are changed such that the sum of the charge level of the toner developing area and the transfer charge falls within an allowable range.
In an image forming apparatus using the electrophotographic method, it is known that due to differences between systems and in the gradation method used, as well as due to differences in environmental conditions, durability, darkness level set by the user, etc., the amount of toner adhering to the photoreceptor per unit area (hereinafter termed simply `adhering toner amount`) is around 0.7 mg/cm.sup.2, and can vary within a range of approximately 0.4 mg/cm.sup.2 to 1.0 mg/cm.sup.2.
FIG. 2 shows the I-V characteristic when a solid toner image is transferred to a regular sheet of A4 size paper by means of the contact transfer method employing a transfer roller. B1, B2 and B3 represent the characteristic when the amount of toner adhering to the photoreceptor is 0.4 mg/cm.sup.2, 0.7 mg/cm.sup.2 and 1.0 mg/cm.sup.2, in that order. The level of toner charge is more or less fixed. The darkened part of each characteristic curve indicates the best transfer efficiency range (hereinafter termed the `proper range`) obtained regarding the adhering toner amount represented by each curve.
Below the proper range, the transfer output becomes too low and transfer problems such as the image becoming blurred occur. Above the proper range, the transfer output becomes too high and transfer problems such as discharge noise occur. This proper range varies depending on the adhering toner amount, as shown in the drawing. FIG. 2 is a drawing showing the characteristic for regular A4 paper immediately after it was taken out of its package, and the characteristic for other types of transfer medium would differ from that shown in the drawing. For example, where the paper has absorbed moisture, even when it is regular paper, the required transfer voltage level is smaller. Where the transfer medium comprises thick paper or an OHP transparency, the resistance level is larger than that for regular paper, and therefore, the necessary transfer voltage level rises.
In the conventional fixed-voltage transfer control method, the transfer voltage is controlled such that it will comprise a control voltage level V2 falling within the proper range when the adhering toner amount is 0.7 mg/cm.sup.2. As a result, if the overall adhering toner amount deviates from 0.7 mg/cm.sup.2 and the resistance of the transfer medium varies, the paper assumes a characteristic different from the B2 characteristic shown in FIG. 2. Consequently, the transfer voltage level controlled to be voltage V2 falls outside the proper range for this different characteristic, and as a result, good transfer efficiency can no longer be obtained. In other words, while the conventional fixed-voltage transfer control method has little susceptibility to localized non-uniformity with regard to the image pattern and B/W ratio (black/white ratio), it is easily affected by the overall change in resistance of the transfer medium which varies depending on the type of the transfer medium or in response to the environment.
The conventional fixed-current control method is capable of performing control even when the adhering toner amount generally deviates from 0.7 mg/cm.sup.2 or when the overall resistance of the transfer medium changes. However, in the case of FIG. 3(b), where a blank area and a patterned area coexist within the range of the transfer nip, i.e., when localized non-uniformity exists, more charge flows to the blank area as shown in the drawing, and consequently, the charge that should flow to the patterned area falls short, which easily leads to transfer failure. If the control current level is set high in order to prevent this problem, excessive charge flows when the solid area is transferred, which leads to the occurrence of pre-transfer discharge and therefore, discharge noise in the image. In other words, while the conventional fixed-current transfer control method has little susceptibility to an overall change in resistance, it is easily affected by a localized change in resistance caused by the image pattern or the B/W ratio, which easily leads to poor transfer efficiency. Such a problem is particularly marked during color image formation where the toner layer becomes thick. FIG. 3(a) is showing the flow of charge in a solid image area.